Data_Ghost
She steals. Obviously. But she's on the run from many different people, from many different things, and from River and The Doctor. Most importantly, The Doctor. She needs to keep him on his toes and in the dark. If he finds out what else she's doing he may figure it out before her, and that's what she doesn't want.
So she'll keep leading him on for now. Him and River. She'll be the thief they can't catch, the troublemaker they come to slowly know.
***Warning. Some language, violence, manipulation, mention of suicide, and murder.***